1. Field of the Invention
The present invention relates to a magnetoresistive element and a method of manufacturing the magnetoresistive element.
2. Description of the Related Art
A magnetoresistive random access memory (MRAM) is a nonvolatile semiconductor memory which stores data using a variation in resistance of a magnetoresistive element due to a change in relative relationship between directions of magnetization of two ferromagnetic layers included in the magnetoresistive element.
A memory cell of an MRAM includes a magnetoresistive element in which a plurality of ferromagnetic layers and a barrier layer are laminated. The magnetoresistive element comprises a ferromagnetic layer whose magnetization direction is pinned (hereinafter referred to as a “magnetic pinned layer” or a “reference layer”) and a ferromagnetic layer whose magnetization direction is variable (hereinafter referred to as a “magnetic free layer” or a “storage layer”). By varying the magnetization direction of this magnetic free layer, data is written.
In an MRAM in which data is written using a magnetic field generated by a current flowing through an interconnect (magnetic field write type MRAM), if the size of a magnetoresistive element is reduced, the coercive force between ferromagnetic layers is increased, leading to a tendency of increasing a current required for writing. With the magnetic field write type MRAM, it is difficult both to achieve miniaturization of a memory cell toward an increase in capacity and to reduce power consumption of the memory cell.
As a write method to overcome such a problem, a spin transfer MRAM using the spin transfer torque (STT) write method has been proposed.
In the spin transfer MRAM, writing of data is carried out by passing a spin-polarized current having a given value or more from one end of a magnetoresistive element to the other end, or from the other end of the magnetoresistive element to one end, to vary the direction of magnetization of the magnetic free layer.
In a method of manufacturing an MRAM, constitution members, interconnects and electrodes of a magnetoresistive element are processed using, for example, ion milling.
Ion milling is a method of processing a member by physical sputtering. Upon the processing, the processed substance sometimes adheres as a residue to a constitution member of the magnetoresistive element. If the residue is electrically conductive, the residue causes a short circuit between two ferromagnetic layers, which sandwich a tunnel barrier layer, and the tunnel barrier layer. This is a cause of failure of the magnetoresistive element and an MRAM using it. Particularly in a spin transfer MRAM, a current is passed through a magnetoresistive element, and therefore the current flows between two ferromagnetic layers via the conductive residue to make writing of data impossible.
An example of techniques to solve this problem is disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2005-44848. In Jpn. Pat. Appln. KOKAI Publication No. 2005-44848, however, an effect of removing the residue from a side surface of a magnetoresistive element is not sufficiently obtained unless a taper angle formed between the side surface and the bottom surface of the magnetoresistive element is 60° or less. As a result, the size of the formed magnetoresistive element cannot be reduced. Arranging a plurality of magnetoresistive elements with high density becomes difficult.